The present invention relates to cylinder liners and mold assembly techniques for alloy engine blocks.
It is common practice in the automotive and engine-manufacturing industry to keep the weight of the component parts of a vehicle to a minimum if possible, as this has benefits associated with both the handling and fuel consumption of the completed vehicle. Thus while engine blocks were for long periods cast from cast iron, the cylinder bores were carefully machined to receive the pistons of the engine. However with the introduction of aluminium cylinder blocks, it is necessary to have in the cylinder block an iron cylinder liner in which the piston of the engine reciprocates, due to the fact that aluminium is not a sufficiently wear resistant metal.
When assembling a mold for an aluminium engine block, the cylinder liners are generally positioned on a core known more specifically as a barrel core, which is integrally formed with the crankcase core. The cylinder liners are then preheated using induction heaters. The remainder of the mold is then assembled around the integral barrel and crankcase core, and the liners, and the mold is then filled, casting the iron liners into the engine block. The positional accuracy then of the bore liners relative to one another within a casting is determined in a large part by the dimensional accuracy and assembly clearances of the mold cores that support the bore liners during the filling of the mold.
The liners are preheated prior to casting in order to improve molten metal flow over the liners during casting; this in turn improves the quality and the mechanical and thermal properties of the completed casting.
A problem then with molds where the barrel core is integrally formed with the crankcase core is that the cylinder liners must be positioned on the barrels very early in the mold assembly sequence, as a result the liners can cool considerably between the time when they are preheated, and the time when the mold is filled with molten metal, due to the amount of mold assembly still required post heating.
Once cast, an engine block is typically put through a series of machining processes; one of these machining processes involves machining the internal diameter of the cylinder liners to ensure that the cylinder liners have uniform wall thickness, and a constant internal diameter along their length. This places further importance upon positional accuracy of the liners during casting.
Compounding this problem is that fact that, as a byproduct of its manufacture, a barrel core is formed with a draft, or external diametral taper, in order to permit removal of the core from the core box tooling once formed.
Consequently, when a conventional liner is disposed upon a barrel core, there is a mismatch, which increases along the length of the liner. Although the liner is located tightly at one end of the barrel core, at the distal end of the barrel core, the mismatch is it's most pronounced, permitting a degree of movement, which can potentially result in a misalignment of the liner in the casting.
It is an object of the present invention then to provide a cylinder liner, and mold assembly technique that overcomes or at least substantially ameliorates the problems associated with the cylinder liners and mold assembly techniques of the prior art.
Other objects and advantages of the present invention will become apparent from the following description, taken in connection with the accompanying drawings, wherein, by way of illustration and example, an embodiment of the present invention is disclosed.